In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required. This includes the width and spacing of interconnecting lines and the surface geometry such as comers and edges of various features. Since numerous interconnecting lines are typically present on a semiconductor wafer, the trend toward higher device densities is a notable concern.
The requirement of small features, such as metal lines, with close spacing between adjacent features requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as optical light, X-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the photomask, for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Exposure of the coating through the photomask causes a chemical transformation in the exposed areas of the coating thereby making the image area either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. However, lithography is not without limitations. Patterning features having dimensions of about 0.25 .mu.m or less with acceptable resolution is difficult at best, and impossible in some circumstances. This is because photoresist layers used in lithography have thicknesses on the order of 7,000 .ANG. and higher. Such relatively thick photoresist layers are not conducive to making small patterned dimensions with good resolution. In other words, critical dimension control for features having dimensions of about 0.25 .mu.m or less is poor with relatively thick photoresist layers.
However, corner rounding problems and insufficient etch protection are associated with using thinner photoresists (less than 7,000 .ANG.). In particular, comer rounding of layers underneath thin photoresists is caused by insufficient etch protection and results in poor definition/resolution. Improved lithography procedures providing improved resolution and improved critical dimension control are therefore desired.